Based on how things go, if the format can accept a midrange Warlock deck I think this card will make the cut, even if its just as a Blessing of Kings for 5 with sometimes important upside. As removal this is definitely mediocre, but not unplayable since 5 is enough to kill a large variety of cards, at least atm. If you have enough targets we all know how insane Kings can be, and this could easily be that. ATM the deck this would go in doesn't exist outside of fringe play.

As a removal is pretty bad, since Syphon Soul costs 1 more and kills any minion, even with divine shield. As a buff, i guess i whould preffer Power Overwhelming or Demon Fire. First because costs 1 or 2, and i will usually use for trade or activing deathrattle. Second, because a minion buffed with Felheart will probably get BGH or whatever. Felheart is quite slow to the actual meta. And even with new demons, guess i will still preffer demon fire. More potential on the early game and can avoid BGH most of times. Some pros is that kills first sludge, sylvanna, can be used with cost 5 minions in the same turn (sludge, faceless, etc..)